The human cornea is an independent organism that obtains its sustaining oxygen from the tears of the eye. The oxygen consumption rate of the human cornea is approximated to be 2.8 ml/cm.sup.2 -hr. This value has been determined by Jauregui and Fatt, "Estimation of the Vivo Oxygen Consumption of the Human Corneal Epthelium" in the Americal Journal of Optometry and Archives of American Academy of Optometry, June, 1972, page 507.
Contact lenses have been made from various materials since the early 1950's. Table I illustrates some of the materials, the common term, its composition and its disadvantages.
TABLE I ______________________________________ COMMON COMMON TERM COMPOSITION DISADVANTAGES ______________________________________ Hard 95% + Polymethyl- Very low gas methacrylate Permeability Semi-Rigid 90% + Cellulose Poor Stability Acetate Butyrate Soft 60% + Poly- Poor Durability hydroxyethyl Metacrylate PMMA/Silicone 40%-60% Poly- Marginal oxygen methylmethacrylate permeability & 40%-60% Siloxanyl stability alkyl ester Silicone 10%-90% Phenyl- Poor surface siloxanes and wettability & 10%-90% Methyl- poor durability vinylsiloxanes ______________________________________
Attempts have been made in correcting these disadvantages. Some of these attempts have been somewhat successful, but not entirely. In U.S. Pat. No. 4,152,508, they deal with stability, but the increasing of oxygen permeability was never really attained, due to the use of non-oxygen permeable crosslinking agents. The use of dimethacrylates (the preferred embodiments) did improve the stability over that of U.S. Pat. No. 3,808,178, but did decrease oxygen permeability to some degree. Thus, a material with excellent oxygen and carbon dioxide permeability is preferable for metabolic action of the cornea. The surface of the material must be wettable and compatible with fluids of the eye.